On the outside or from the outside; not internally or internally, usually referring to something applied to the surface of something.
From Latin 'externus' meaning 'outer' or 'external,' combined with '-ly' to make an adverb. The Latin root comes from 'exterus' (comparative: 'exterior') related to being 'outside.' Medical and scientific use became common from the 1600s onward.
You'll see 'externally' all over medicine bottles and instructions ('apply externally only') because doctors needed precise language to warn people not to drink something meant only for skin—this precision in medicine shaped how we use the word today.
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